Why does my material only render correctly on certain faces?
You’re using UV coordinates to sample your Voronoi texture. This is fine, but the default UV map will not automatically compensate for stretching/scaling of the model. To illustrate, I took a cube and shrunk it along one axis:

Note how in the UV map, all six faces are still squares of equal area, even though they aren’t on the model.
Fortunately, all you need to do to fix this is to re-unwrap the mesh. If you don’t care about continuity between the faces at all, you can do the simplest thing of marking all edges as seams by selecting the whole mesh with A, then CtrlE ▸ Mark Seam. Then you can do U ▸ Unwrap, which will automatically compensate for face size:

As you can see, this fixes the texture stretching.